A laser diode is a semiconductor device in which, when its p-n junction is sufficiently forward biased, holes and electrons recombine and photons are emitted. Above a threshold current stimulated emission can occur, resulting in a directional beam of coherent output radiation. The wavelength of the output radiation depends upon the dimensions of the laser diode. The amplitude of the output radiation depends on the magnitude of the forward current. Thus by altering the forward current through the laser diode, modulation of the power output of the diode may be achieved. There is an upper limit to the current which may be applied to the laser diode which, if exceeded, may cause excessive heating and could result in its destruction.
The threshold current is temperature dependent and thus the output power may vary even when the forward current through the diode is constant, with variation in external and internal temperatures. This can cause problems when it is wished to modulate the power output for data communication. Similar considerations apply to L.E.Ds, although in this case the threshold level is not precisely defined, but may be taken to be the lowest level at which a useful light output is produced.
One way in which compensation for the thermal instability of the threshold current may be achieved is to sample the output of a laser diode using a photodiode to provide a feedback signal to control the forward current. However, this solution is not suitable where it is desired to amplitude modulate the laser diode output at a reasonably high frequency, for example, where the laser diode is used in an optical communication arrangement.
In a previous method for attempting to overcome the problem of thermal stability while still being able to amplitude modulate the laser diode at a sufficiently high frequency, the current through the laser consists of two components. One component, known as the center point current, is arranged to forward bias the laser diode to a center point approximately midway between the threshold current and the current giving a maximum safe output power at a given temperature. This component is made to vary with diode junction temperature, by using a photodiode to give a feedback control signal, thus enabling it to remain at the midpoint. The other current is known as the modulating current and, when combined with the centre point current, enables the output of the laser diode to vary from the threshold condition to a maximum safe output.